One important application is the measurement of extremely high voltage. Since the glass fibers are intrinsically insulating, there's a safe isolation from the voltage source, and the voltage is sensed by the Kerr effect, the change in the refractive index in response to an applied electric field, in the fiber material. While the prior examples are sensors that utilize an intrinsic property of the optical fiber, fibers can be combined with other elements to form sensors. Many years ago I worked on the combination of magneto-opticalgarnet and optical fibers to create a variety of magnetic sensors for aerospace applications. These included simple switches and wheel speed sensors.

Says Zhengyong Liu, the lead author of the paper describing this work, “An all-optical sensing network has many advantages as it is immune to electromagnetic interference, has long transmission distance and the sensors don't require electricity... However, there is a need for fiber-optic sensors that are optimized to measure different parameters in railway systems."[6]

The Hong Kong fiber-optic accelerometer is based on a polarization-maintainingphotonic crystal fiber that's formed into a disk shaped coil just 15 millimeters in diameter. This coiled fiber is glued onto a stainless steelsubstrate, and a cylindricalmass is affixed above it. Vibrations cause the mass to press the fiber, and this causes a phase shift in its transmitted light that's read by an interferometer (see figure).[5-6]

The Hong Kong team designed and fabricated the sensing fiber to have airholes to provide the photonic crystal structure, and also to have a small diameter, low bending loss, and high birefringence.[6] The resulting accelerometer sensor has double the response time of conventional fiber sensors that incorporate a Bragg grating as the sensing structure.[6] The fiber-optic sensor has no moving parts, and it is immune to the electrical noise that exists in the high-voltage train environment.[6]

Field tests were done on an in-service train in comparison with a Bragg grating fiber accelerometer and a piezoelectric accelerometer.[6] The accelerometer sensor based on the polarization-maintaining photonic crystal fiber had a sensitivity of ~8 pm/g, and a resonant frequency exceeding 2.5 kHz.[5] The high frequency response allows detection of the impulse associated with a defect or crack present on the railway track.[5]